1. Field of the Invention
The present invention relates to a padding material for acoustic applications. In particular, the present invention relates to a gradient density padding material, wherein the density of a portion of the thickness of the padding material is increased to increase the air flow resistance of the padding material. In view of this, the acoustic properties of the padding material can be tuned to a particular application.
2. Description of Background Art
There is a growing trend in today's acoustical insulation market for lightweight materials. The transportation industry is a good example of where there is a need for lightweight acoustical insulation. Traditional interior and exterior acoustical insulators have used heavy layers in decoupled mass systems to reduce interior noise levels. A recent trend in this market has been to eliminate these heavy layer systems and use lightweight fiber or foam based padding systems. There are a variety of these materials out in the market today. All of these fiber systems utilize a multilayer fiber pad to produce a lightweight padding system with good acoustic absorption in the range of 100-5000 Hz and little or no sound level degradation in the interior of a vehicle when compared to heavy layer systems. It is known in the acoustic field that acoustic absorption performance is dependent in part on the airflow resistance of materials. It is also known that by increasing the airflow resistance of an acoustic material, the low to mid frequency (100-2500 Hz) absorption performance can be increased. Maximum acoustic absorption in this frequency range occurs when that material has an airflow resistance of 800-1200 MKS Rayls. This frequency range is often the most difficult to combat in vehicle systems where weight, cost, and packaging space (thickness) are limited. All of today's multilayer systems require 2 or more separate layers, where each layer is produced independently with specific stiffness, strength, and airflow resistance properties. These independent layers must then be bonded together in a secondary operation.
One example of a multilayer system according to the background art is described in U.S. Pat. No. 6,145,617. In this patent, a compressed nonwoven fiber layer is bonded to an uncompressed fiber layer. A second example is described in U.S. Pat. No. 6,296,075. In this patent, a high flow resistant facing or coverstock material is attached to the surface of a low density fiber web. A third example is described in U.S. Pat. No. 5,824,973. In this patent, a low density fiber web includes a microporous film attached to a surface thereof. A fourth example is described in U.S. Pat. No. 5,334,338. In this patent, a dense foam layer with low open cell count is produced on top of a porous, low density open cell foam layer.